1. Field of the Invention
The present invention relates to a zinc oxide sputtering target and a method for producing the same.
2. Description of the Related Art
For a transparent conductive film used in electronic devices and the like, indium tin oxide (ITO) and the like have been widely used over many years. However, in view of the situation where the prices of rare earth metals such as indium have soared in recent years, rare metal alternatives are strongly desired. Recent vigorous attempts to prepare a transparent conductive film with less expensive zinc oxide (ZnO) have not genuinely come into practical use as having various problems such as failure to attain a desired conductivity due to difficulty in sufficiently lowering electrical resistivity.
Meanwhile, it is the industrial mainstream that a transparent conductive film is produced by sputtering. A sputtering target reaches a high temperature during sputtering, hence the sputtering target is cooled by a backing plate having water channels inside. Thus, a temperature difference between the sputter surface exposed to a high temperature and the water-cooled backing plate surface tends to generate heat stress inside the sputtering target, which may cause cracks in the sputtering target.
Patent Document 1 (JP2009-215629A) discloses that generation of cracks can be suppressed if a coefficient of linear thermal expansion in the direction perpendicular to the sputter surface of the target is more than 10% greater than that in the direction parallel the sputter surface. However, the effect of suppressing the occurrence of cracks was not sufficient when the power supply was increased for achieving higher productivity during sputtering.
Patent Document 2 (JP3128861B) and Patent Document 3 (JP3301755B) disclose a sputtering target composed of a zinc oxide sintered body. Although these documents disclose that the quality of the sputtered film can be improved by having zinc oxide oriented along with the (101) or (002) plane, such improvement was not intended to suppress the occurrence of cracks in the target.